The invention relates to generation of short optical pulses with particular application to transmission of data.
Ultra high speed time domain multiplexing (TDM) optical transmission systems in optical fibers require compact light emitting sources capable of generating optical short pulse trains in a picosecond/sub-picosecond range. General requirements for short pulse sources such as soliton sources are narrow pulse width, low time jitter and a continuously tunable repetition rate. For practical fiber optic systems, there are additional requirements of long-term reliability, small size and easy data encoding in the system application.
There are several known methods to generate short optical pulse trains. Complicated passive or active mode locking techniques are available for high speed optical pulse generation where pulses are generated at a fixed repetition rate determined by the roundtrip time of the laser resonator, e.g. D. J. Derickson et al., xe2x80x9cShort pulse generation using multisegment mode-locked semiconductor lasers,xe2x80x9d IEEE J. Quantum Electron., Vol. 28, pp. 2186-2202, 1992. These techniques are sensitive to phase matching conditions and therefore difficult to build and maintain. Another method is gain switching of lasers which suffer from high time jitter. Pulse generation at repetition rates over 50 GHz is extremely difficult to achieve in this method because of limitations of the device modulation bandwidth and radio frequency supply as described, e.g. in publication by A. G. Weber, W. Ronghan, E. H. Bottcher, M. Schell and D. Bimberg, xe2x80x9cMeasurement and simulation of the turn-on delay time jitter in gain-switched semiconductor lasers,xe2x80x9d IEEE J. Quantum Electron., Vol. 28, pp. 441-445, 1992.
High repetition rate optical pulses can also be generated using a dual wavelength light source as described, e.g. in publication P. V. Mamyshev, S. V. Chernikov and E. M. Dianov, xe2x80x9cGeneration of Fundamental soliton trains for high-bit-rate optical fiber communication lines,xe2x80x9d IEEE J. Quantum Electron., vol. 27, pp. 2347-2355, 1991. Two wavelengths emitted by two lasers are mixed to form a high frequency sinusoidal signal which is sent though an optical combiner and optical amplifier followed by a nonlinear fiber. As a result the sinusoidal signal is compressed into a train of optical pulses. The common approach of dual wavelength light sources is to use two discrete lasers, which is complex and suffers from the long term stability issue. Dual wavelength operation can also be accomplished by selecting the appropriate phase modulation sidebands from an externally phase modulated light source, e.g. P. V. Mamyshev, xe2x80x9cDual-wavelength source of high-repetition-rate, transform-limited optical pulses for soliton transmissionxe2x80x9d Opt. Lett., Vol. 19, pp. 2074-2076, 1994. This method requires high frequency modulation and optical filters. In order to make this method more practical two solitary laser diodes are usually used to generate the sinusoidal beat signal. Unfortunately, frequency variations of each laser are subject to both thermal and mechanical fluctuations which result in beat signal frequency fluctuations. Phase noise of both lasers also contribute to the jitter of the beat signal significantly. The control of the polarization from each laser output and the effort to align and maintain them is also a practical issue that decreases the system performance. Therefore, the resulting signal performance is not satisfactory and practical use of such a configuration in commercial ultra high speed applications is in question.
Accordingly, there is a need in the industry for a practical, compact and reliable optical source of continuously tuning high repetition rate short optical pulses which is suitable for optical transmission systems and high speed optical signal processing.
It is an object of the present invention to provide an optical pulse source which avoids the afore-mentioned problems.
Thus, according to one aspect of the present invention there is provided an optical pulse source, comprising:
a first single mode DFB semiconductor laser having a first grating for generating light at a first frequency;
a second single mode DFB semiconductor laser having a second grating for generating light at a second frequency;
the lasers having a common active medium and shared optical path, the lasers providing mutual light injection into each other resulting in generation of a beat signal at a difference frequency of two lasers;
an optical compressor disposed to receive the beat signal and compressing the pulse duration of the signal, thus forming a train of short optical pulses having a pre-determined duration and a repetition rate.
The source may further include a saturable absorber disposed to receive the beat signal before it is sent to the optical compressor. The absorber provides an initial time compression of the signal, thus transforming the beat signal into an initial train of optical pulses. Additionally, an optical amplifier may be used for amplification the beat signal or the initial train of pulses, e.g. including an erbium doped fiber. The optical compressor may include a dispersion decreasing fiber, a dispersion shifted fiber and/or an external erbium doped fiber amplifier. The source may further include means for data encoding into the train of short pulses, e.g. an optical modulator operating at a speed determined by the repetition rate. A typical range of the repetition rates is from about several tens GHz to about several hundred GHz with a sub-range from about 25 GHz to about 80 GHz being of special importance for data transmission. A typical duration of pulses in the pulse train is within a picosecond/sub-picosecond range.
A source includes gain coupled DFB lasers, or alternatively it may include loss coupled DFB lasers. Preferably, the active medium of the lasers includes a multiple quantum well structure. Advantageously, the first and second gratings in the first and second lasers are formed by etching grooves directly through the multiple quantum well structure. Beneficially, each grating has a period comprising a first section and a second section with substantially all quantum wells being etched away from the second section, thus providing no substantial photon emission in the second section and ensuring no substantial interaction between the lasers. A source may further have means for stabilizing the frequency of one of the first and second lasers, or means for stabilizing frequencies of both lasers. Means for tuning frequencies of the first and second lasers may be provided additionally. To ensure reliable and accurate mode, low frequency modulation of light generated by one of the first and second lasers, may be provided. Alternatively, light generated by the lasers may be modulated simultaneously. Beneficially, the modulation is provided at a frequency which is subharmonic to the beat frequency.
In the first embodiment a source includes first and second gratings which have same periods, and it is arranged that lasers generate light at the different sides of stopband.
In the second embodiment a source includes first and second gratings which have same periods, and it is arranged that lasers generate light at the same side of stopband. The difference between the first and second frequencies is provided by different current injection into the first and second lasers, or by difference in temperature control of the first and second lasers. Alternatively it may be provided by different width of the active medium in the first and second lasers.
In the third embodiment a source includes the first and second gratings which have different periods.
It is arranged that the frequency of one of the lasers which is remote from an output facet does not fall within a stopband of the other laser which is closer to the output facet so that light emitted by the remote laser can pass through the shared optical path to the output facet.
In the fourth embodiment, instead of pumping the active medium of the source by current injection in to the active medium, the pumping is provided by an external optical pumping source.
In modifications to the embodiments described above, a source may include the first and second gratings which are either uniform or chirped, the gratings preferably being first order gratings which are formed by holographic techniques or electron beam writing onto the active medium.
According to another aspect of the invention there is provided an optical pulse source, comprising:
a first single mode DFB semiconductor laser having a first grating for generating light at a first frequency;
a second single mode DFB semiconductor laser having a second grating for generating light at a second frequency;
the lasers having a common active medium and shared optical path, the lasers providing light injection of light into each other resulting in generation of a beat signal at a difference frequency of two lasers;
a saturable absorber disposed to receive the beat signal and providing an initial time compression thus transforming the beat signal into an initial train of optical pulses;
an optical amplifier disposed to receive the initial pulse train after the absorber;
an optical compressor disposed to receive the pulse train after the optical amplifier and compressing duration of pulses of the train, thus forming a train of short optical pulses having a pre-determined duration and a repetition rate,
the first and second lasers, the saturable absorber and the optical amplifier being formed on the same chip.
Conveniently, the lasers, the saturable absorber and the optical amplifier are integrated within a package. According to yet another aspect of the invention there is provided a source of radiation, comprising:
a first single mode DFB semiconductor laser having a first grating for generating light at a first frequency;
a second single mode DFB semiconductor laser having a second grating for generating light at a second frequency;
the lasers having a common active medium and shared optical path and providing mutual light injection into each other resulting in generation of radiation at a beat frequency of two lasers.
Preferably, the source of radiation includes either gain coupled DFB lasers or loss coupled DFB lasers with the active medium comprising a multiple quantum well structure. Advantageously, the first and second gratings are formed by etching grooves directly through the multiple quantum well structure. To provide no substantial interaction between two lasers, it is arranged that each grating has a period comprising a first section and a second section with substantially all quantum wells being etched away from the second section, thus providing no substantial photon emission in the second section. A typical wavelength range of the radiation generated by the source corresponds to microwave to millimeter wavelength range.
Conveniently, the first and second gratings have same periods, and it is arranged that the first and second lasers generate light at the same side of stopband. The difference between the first and second laser frequencies may be provided by different current injection into the first and second lasers, by different width of the active medium in the first and second lasers and/or by difference in temperature control of the first and second lasers.
Alternatively, a source of radiation may include gratings having same periods while the first and second lasers generate light at different sides of stopband. In yet another alternative a source may include the first and second gratings having different periods. It is also provided that the frequency of one of the lasers which is remote from an output facet does not fall within a stopband of the other laser which is closer to the output facet so that light emitted by the remote laser can pass through the shared optical path to the output facet. Conveniently, the source may further include means for modulation of light generated by one of the first and second lasers at a frequency which is subharmonic to the beat frequency. Advantageously, the source is formed on a chip and integrated within a package.